The present invention relates to a process for the production of sodium chloride crystals.
A widely used technique for producing solid sodium chloride consists in crystallizing it by evaporation of a sodium chloride brine. The sodium chloride brine used in this technique can be, for example, seawater. In an alternative form, it can be obtained by dissolution of rock salt in water. The brines used in this technique for the manufacture of sodium chloride are usually contaminated by undesirable compounds which risk contaminating the sodium chloride. This is in particular the case with potassium chloride, sodium carbonate and sodium sulphate which, if they are not removed from the brine before evaporation of the latter, could easily be encountered in association with the sodium chloride crystals. The removal of these undesirable compounds from the brine moreover presents the problem of their discharge and of their storage (generally in public landfill sites) under conditions suited to preventing them from polluting the environment.
A process for the manufacture of sodium chloride crystals starting from a sodium chloride brine contaminated by potassium chloride and sulphate ions is known (U.S. Pat. No. 4,547,197). According to this known process, calcium hydroxide is added to the brine, to precipitate the sulphate ions in the form of calcium sulphate, and then sodium carbonate is added, to precipitate the calcium ions in the form of calcium carbonate. The aqueous solution collected after separation of the precipitates is subjected to evaporation at a temperature of approximately 120xc2x0 C. to crystallize sodium chloride, which is collected, and the aqueous mother liquor from the crystallization is cooled to approximately 45xc2x0 C. (preferably by subjecting it to a reduction in pressure) to crystallize simultaneously sodium chloride, potassium chloride and sodium sulphate. In this known process, the coprecipitation of the three salts (sodium chloride, potassium chloride and sodium sulphate) causes difficulties in the discharge of the latter. A solution suggested in the document U.S. Pat. No. 4,547,197 consists in dispersing the mixture of the three salts in cold water, so as to selectively dissolve the sodium chloride and the potassium chloride, while the sodium sulphate recrystallizes in the form of Glauber""s salt.
The abovementioned known process exhibits a disadvantage of great complexity, imposed by the need to dispose of the mixture of the three salts (sodium chloride, potassium chloride and sodium sulphate) without harming the environment. This known process exhibits the additional disadvantage that it consumes a large amount of calcium compound (calcium hydroxide) in extracting the sulphate ions from the brine.
The invention is aimed at overcoming the disadvantages of the known process described above by providing a process for the manufacture of sodium chloride crystals from a brine contaminated by potassium chloride and sulphate ions which makes possible a substantial reduction in the consumption of calcium compound in extracting the sulphate ions from the brine and which, moreover, simplifies thee discharge and the storage of the residual solid materials from the purification.
The invention consequently relates to a process for the production of sodium chloride crystals from a sodium chloride brine contaminated by potassium chloride and sulphate ions, according to which, in a first stage, a calcium compound is added to the brine to precipitate calcium sulphate, which is isolated, and an aqueous solution is collected, in a second stage, the aqueous solution from the first stage is subjected to evaporation, to crystallize sodium chloride, and sodium chloride crystals and an aqueous mother liquor are collected separately, and, in a third stage, the aqueous mother liquor from the second stage is subjected to cooling in order to crystallize at least a portion of the potassium chloride; according to the invention, the amount of calcium compound in the first stage and the cooling in the third stage are adjusted so that the sulphate ions precipitate in the form of glauberite in the first stage and of glaserite in the third stage.
In the process according to the invention, the sodium chloride brine is, by definition, an aqueous sodium chloride solution. It is invariably an aqueous solution which is saturated or unsaturated with sodium chloride. Its content by weight of sodium chloride is advantageously greater than 5%, usually at least equal to 10%. Brines comprising at least 20% by weight of sodium chloride are especially recommended. Brines which are substantially saturated at ambient temperature are preferred.
The sodium chloride brine employed in the process according to the invention is contaminated by impurities. These impurities comprise potassium chloride and sulphate ions. The sulphate ions are, for example, present in the form of dissolved sodium sulphate. The impurities are normally present in brine in an amount of less than the sodium chloride content.
In the first stage of the process according to the invention, the role of the calcium compound is to react with the sulphate ions to form calcium sulphate, which crystallizes. The calcium compound must consequently be chosen from those which are capable of reacting with sulphate ions, in particular with alkali metal sulphates (in particular sodium sulphate), to form calcium sulphate. The calcium compound employed in the first stage is advantageously a water-soluble compound. Calcium chloride is preferred.
On conclusion of the first stage, the calcium sulphate crystals are isolated from the brine. The means employed for this purpose is not critical. It advantageously comprises a filtration or a sedimentation followed by a separation.
In the second stage of the process according to the invention, the aqueous solution collected after separation of the crystals from the first stage is subjected to evaporation in order to crystallize sodium chloride. The parameters of the evaporation (in particular temperature, pressure and the degree of evaporation) are chosen so as to avoid simultaneous crystallization of undesirable compounds, such as potassium chloride or sodium sulphate. The optimum values of these parameters will depend on the concentration of the brine, on its contents of potassium chloride and of sulphate ions and, if appropriate, on the other impurities present. They can be easily determined by routine work, from liquid-solid equilibrium diagrams, in particular the Naxe2x80x94Kxe2x80x94Clxe2x80x94SO4xe2x80x94H2O diagram, accessible from the information in the literature.
On conclusion of the second stage, the sodium chloride crystals are separated from the aqueous mother liquor. The said separation can be carried out by any appropriate means, for example by filtration, by centrifuging or by sedimentation followed by separation.
In the third stage of the process according to the invention, the aqueous mother liquor from the second stage is subjected to controlled cooling in order to render insoluble and to crystallize the potassium chloride.
In accordance with the invention, the calcium compound is employed in the first stage in an amount which is in deficiency with respect to that which is necessary to convert all the sulphate ions to calcium sulphate. More specifically, the amount of calcium compound employed in the first stage is adjusted according to the potassium content of the brine, so that a fraction of the sulphate ions of the brine precipitates in the form of glauberite (mixed calcium sulphate and sodium sulphate compound of general formula CaSO4.Na2SO4) in the first stage and so that the balance of the sulphate ions precipitates with all the potassium ions in the form of glaserite (mixed potassium sulphate and sodium sulphate compound of general formula Na2SO4.3K2SO4) in the third stage. The removal of the sulphate ions and potassium ions by successive crystallizations of glauberite and glaserite is based on a sulphate content in the brine which is greater than that necessary to precipitate all the potassium ions in the form of glaserite. In practice, the brine should consequently comprise a molar amount of sulphate ions of greater than 1.5 times its molar content of potassium ions. The parameters of the process in the first stage (the temperature and the amount of calcium compound employed), in the second stage (the temperature, the pressure and the degree of evaporation) and in the third stage (the temperature and the pressure) should be determined in each specific case according to the concentration of the brine and according to its respective contents of potassium chloride and of sulphate ions. The optimum values of these parameters can be easily determined by calculation and from the liquid-solid equilibrium diagrams.
In practice, good results are obtained when the evaporation, in the second stage, is carried out at a temperature of greater than 75xc2x0 C. and not exceeding 200xc2x0 C. (temperatures of 80 to 120xc2x0 C. being preferred). The pressure should be adjusted to the temperature selected and can be less than standard atmospheric pressure in the case of low temperatures or greater than the latter in the case of high temperatures. The cooling in the third stage is advantageously carried out at a temperature of less than 80xc2x0 C., preferably of 10 to 70xc2x0 C., temperatures in the region of ambient temperature (for example from 15 to 30xc2x0 C.) being recommended.
It may happen that the aqueous solution collected from the first stage comprises dissolved calcium ions. To this end, according to a specific embodiment of the process according to the invention, the aqueous solution collected from the first stage has sodium carbonate added to it before carrying out the evaporation in the second stage. In this embodiment of the process according to the invention, the role of the sodium carbonate is to react with the residual calcium ions to crystallize calcium carbonate, which is removed by any appropriate means. The amount of sodium carbonate employed in this embodiment of the invention can be easily determined from the residual amount of calcium ions in the aqueous solution.
In addition to the potassium chloride and sulphate ions, the sodium chloride brine subjected to the process according to the invention can optionally comprise other dissolved impurities, in particular sodium carbonate. To this end, according to another embodiment of the process according to the invention, the brine is treated, upstream of the first stage, by any appropriate means for removing the sodium carbonate which it comprises. To this end, in a first alternative implementation of this embodiment of the process, the brine is treated, upstream of the first stage, with hydrochloric acid in order to decompose the sodium carbonate and to form sodium chloride. The carbon dioxide generated by the reaction is discharged from the brine by any appropriate degassing means. In a second alternative implementation, the brine is treated, upstream of the first stage, with carbon dioxide in order to crystallize sodium bicarbonate, which is collected.
The process according to the invention applies well to sodium chloride brines which comprise, per kg of dry matter, from 550 to 800 g of sodium chloride, from 50 to 350 g of sodium sulphate and from 5 to 100 g of potassium chloride, and optionally from 50 to 250 g of sodium carbonate. It finds an application in the treatment of seawater or sodium chloride brines obtained by dissolution of rock salt. The invention is especially suited to waste brines from the purification treatments, by means of basic sodium compounds, such as sodium hydroxide or sodium bicarbonate, of flue gases contaminated by hydrogen chloride. The invention is especially suited to the treatment of the brines produced by dispersing, in water, the residual material which is obtained after purification, by means of a basic reactant selected from sodium carbonate, sodium bicarbonate and sodium sesquicarbonate, of a flue gas originating from the incineration of waste comprising chlorinated compounds (in particular waste of domestic or hospital origin or some types of industrial waste) In this specific application of the process according to the invention, the brine is preferably subjected to a preliminary treatment for purification from heavy metals, for example by applying the technique disclosed in the document EP-B-603 218 [Solvay (Socixc3xa9txc3xa9 Anonyme)].